Method for the preparation of concentrated anion-deficient salt solutions

ABSTRACT

Concentrated anion-deficient salt solutions are prepared of the actinide oxides, PuO 2 , UO 2 , UO 3  and U 3  O 8  by dissolving one or more oxides in an aqueous solution of thorium nitrate at a concentration of 4 molar or greater and at a temperature of 60° C. or more. Anion-deficient salt solutions of actinide metals so produced are useful as starting materials for the manufacture of ceramic nuclear fuel particles by the sol-gel process.

This is a division of application Ser. No. 106,922, filed Jan. 15, 1971,now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to the preparation of concentrated anion-deficientsalt solutions.

Anion-deficient salt solutions are for instance suitable for thepreparation of solid oxide and carbide particles.

For the preparation of spherical particles of ceramic nuclear fuel ananion-deficient solution of uranylnitrate can successfully be used as astarting material.

In the prior art these solutions have been prepared according to thefollowing methods:

(1) By dissolving UO₃ in concentrated uranyl nitrate solutions,

(2) By the extraction of nitric acid from stoichiometric, possiblyslightly acid uranyl nitrate solutions.

These methods show, however, the following drawbacks.

For the purpose of the first method it is necessary to have at one'sdisposal a UO₃ of such a texture that this substance easily dissolves inthe uranyl nitrate solution.

As to the second method it is observed that extraction, whereby nitricacid is withdrawn from a stoichiometric or weakly acid uranyl nitratesolution, can only be applied to dilute uranyl nitrate solutions.Moreover, a special installation is needed for this. After removal ofthe nitric acid the solution obtained has to be brought to the requireddegree of concentration, e.g. by evaporation.

The invention aims at giving improved methods for the preparation of ananion-deficient uranyl nitrate solution. Besides it appeared thatanion-deficient actinide salt-solutions could be prepared according toseveral more methods than was formerly possible.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention one or more actinide oxides as PuO₂, UO₃ orlower uranium oxides than UO₃ are dissolved in a small volume of an acidreacting liquid. The acid reacting liquid consists of a small amount ofa strong acid such as a small amount of concentrated HNO₃, HCl or H₂ SO₄or an aqueous solution of an actinide salt of a strong acid as forinstance UO₂ (NO₃)₂ or Th(NO₃)₄.

Mixtures of the above-mentioned liquids can be used too.

With a small amount of liquid is meant that in case of ananion-deficient uranyl nitrate solution the uranium concentration is atleast 2 molar.

It is possible to incorporate during the preparation or thereafter smallamounts of compounds of other elements in the anion-deficient actinidesalt solution in order to improve the properties of nuclear fuelmaterial prepared from this solution.

By other compounds are meant water soluble boron, yttrium, rare earthmetals and zirconium compounds.

Examples of the preparation of mixed anion-deficient actinide saltsolutions are the dissolving of PuO₂ in uranyl nitrate solution and ofUO₃ in thorium nitrate solution.

It has surprisingly been found that anion-deficient solutions of therequired nitrate/actinide metal ratio can be obtained by causing loweroxides than UO₃ to react with strong nitric acid, uranyl nitratesolution, thorium nitrate solution or mixtures of these substances inthe quantities calculated on the basis of the requirements.

The use of lower uranium oxides than UO₃ has the advantage of bettersolubility in acid solutions than UO₃. The difficulty of preparing a UO₃with a suitable texture namely can be avoided.

Lower uranium oxides than UO₃ are the compounds U₃ O₈ and UO₂. Theseoxides, along with uranyl nitrate, are the forms in which uranium isobtainable as a basic material. They are also the forms in which uraniumis preferably conveyed.

It is therefore of importance to convert these oxides in the eaiestpossible manner into the solution required for the process to beemployed.

The required anion-deficient uranyl nitrate solution may becharacterized as follows: ##EQU1##

It is observed that this uranium concentration is higher than that ofthe saturated stoichiometric uranyl nitrate solution.

For the preparation of ceramic fissile material a solution of this kindis first mixed with ammonia-liberating agent and then solidified bybeing dispersed in a phase of sufficiently high temperature,non-miscible with water. With this method it is of great importance tostart with highly concentrated uranium solutions.

In order to make the rate of solution of the uranium oxide in nitricacid as high as possible, it is important to prepare the U₃ O₈ byheating in an oxidizing atmosphere, such as air or oxygen, attemperatures between 600° and 900° C. At these temperatures the mostvolatile and/or combustible impurities are removed and the texture ofthe material is still conducive to solution.

Difficultly soluble UO₂ is likewise converted by this thermal processinginto easily soluble U₃ O₃.

Very difficultly soluble UO₂ is converted into U₃ O₈ by being sinteredin air at 700° C. The cubic lattice of UO₂ is thereby changed into heothorhombic lattice of U₃ O₈. As the molecular volume of U₃ O₈ isgreater than that of UO₂, since UO₂ is of higher density than U₃ O₈, theparticles are completely crumbled. The high specific surface areas ofthe U₃ O₈ obtained in this way has the effect that it can now be readilydissolved in HNO₃.

The preparation of U₃ O₈ as described above is the ideal method ofutilizing waste obtained in the preparation of the ceramic fissilematerial. For this purpose the waste may consist either of unsinteredwaste material, possibly containing organic filter material, or ofsintered final product composed of UO₂.

In accordance with the undermentioned gross equations (1) and (2), thequantities of nitric acid used can be determined by calculation.

    2UO.sub.2 + 5HNO.sub.3 →                                               2{UO.sub. 2 (NO.sub.3).sub.1.5 (OH).sub. 0.5 } + 2H.sub.2 O + NO+NO.sub.      2                            (1)                                              2U.sub.3 O.sub.8 + 11HNO.sub.3 →                                       6{UO.sub.2 (NO.sub. 3).sub.1.5 (OH).sub.0.5 } + 4H.sub.2 O                                                 (2)O+NO.sub.2                                

The invention is further elucidated below by reference to a number ofexamples.

Example I deals with the preparation of an anion-deficient uranylnitrate solution by dissolving UO₂ powder in nitric acid.

Example II deals with the processing of spherical particles ofunsintered UO₃.

Example III deals with the conversion of waste material from sphericalparticles of UO₂ sintered at high temperatures.

Examples IV relates to the dissolving of U₃ O₈ in uranyl nitratesolution.

EXAMPLE I

A solution test was carried out with natural UO₂ powder in nitric acidwith the undermentioned quantities of UO₂ and HNO₃.

weighed-out

    UO.sub.2 :                                                                              11.4854 g.                                                                              =      42.5 mmol of UO.sub.2                              HNO.sub.3 :                                                                             3 × 42.5                                                                          =      127.5 mmol of HNO.sub.3,                       

diluted with water to 100 ml. In this example UO₂ was added in portionsto the hot (˜80° C.) HNO₃ solution.

On account of the fact that during solution in an open beaker somelosses of nitric acid occurred, slightly more nitric acid was used thanwas equivalent to equation (1).

The solution obtained was found to have an NO₃ '/U ratio of 1.6.

EXAMPLE II

A quantity of spherical particles of UO₃ was heated slowly in air to700° C. and then kept at this temperature for another four hours. Thefollowing was obtained: ##EQU2##

This quantity was added in portions to a heated HNO₃ solution consistingof 160 ml. of concentrated HNO₃ (14.4 M) and 258 ml. of water in abeaker. The total volume amounted to 160+258= 418 ml., so that aftersolution the uranium concentration is about 3 M.

The HNO₃ /U₃ O₈ ratio used= 2300/415≈5.5. According to the grossequation (2) an NO₃ '/U ratio ≦1.5 may be reckoned with.

Analysis of the solution obtained gave the following results: ##EQU3##density 1.965 g./cm.³ (20.6° C.).

The solution tests were repeated with two quantities of sphericalparticles of UO₃ with a 20% and 40% enrichment respectively, after theyhad first been converted into U₃ O₈.

The results obtained in this way are set forth below in Table A.

It was observed that by operating in a three-necked flask with a refluxcooler the nitrous vapors had reformed a quantity of HNO₃.

                                      TABLE A                                     __________________________________________________________________________                                           [U]                                    Degree of                         Density,                                    enrich-                                                                             U.sub.3 O.sub.8,                                                                   G. mol                                                                             Ml. HNO.sub.3,                                                                       Mol HNO.sub.3 /                                                                      H.sub.2 O,                                                                        g./cm..sub.3,                                                                      Meas-                                                                             Calcu-                                                                            [NO.sub.3 ']/                  ment  grams                                                                              U.sub.3 O.sub. 8                                                                   14.4M  mol U.sub.3 O.sub.8                                                                  ml. 21°C.                                                                       ured                                                                              lated                                                                             [U]                            __________________________________________________________________________    20%   770.1                                                                              0.917                                                                              350    5.50   500 1.866                                                                              2.82                                                                              2.82                                                                              1.76                           40%   622.3                                                                              0.743                                                                              270    5.25   300 1.904                                                                              2.95                                                                              2.97                                                                              1.58                           __________________________________________________________________________

EXAMPLE III

644.1 grams of spherical particles of UO₂ (sintered at 1400° C. in anatmosphere containing hydrogen), were slowly heated to 750° C. and thenkept for four hours at this temperature. In this way 662.5 grams of U₃O₈ were obtained, which could readily be passed into solution accordingto the method indicated in Example II.

EXAMPLE IV

In this example a quantity of 116 g. of

    UO.sub.2 (NO.sub.3).sub.2.sup.. 6H.sub.2 O

was dissolved in 72 ml. of water and then boiled under reflux with 13.7g. of U₃ O₈ for 21/2 hours.

The clear solution obtained had a 2.49 molar content of uranium and anNO₃ '/U ratio of 1.62.

What is claimed is:
 1. A method for preparing a concentratedanion-deficient actinide salt solution containing at least one actinideoxide selected from the group consisting of PuO₂, UO₂, UO₃ and U₃ O₈,said method including dissolving at a temperature of at least 60° C.,said .[.salt.]. .Iadd.oxide .Iaddend.in an aqueous solution of thoriumnitrate having a concentration of at least 4 molar.
 2. A method for thepreparation of a concentrated anion-deficient actinide nitrate solutionwherein at least one member selected from the group consisting ofuranium dioxide, uranium trioxide and U₃ O₈, is dissolved by stirring ina heated thorium nitrate solution of a temperature of at least 60° C.,and of a concentration of at least 4 molar, and the solution thusobtained is thereafter diluted with water.